Logic Bricks - Stop Sound Actuator
The reason that does not work is because of how the keyboard sensor works, it essentially is stopping and restarting the sound.

What you need to do is have a game property (A boolean set to true). The keyboard sensor is plugged into a Property actuator set to Toggle.

It is then that property that controls the sound actuator. Add a Property sensor, set its Value to "True", the Property to the game property created earlier, and leave the Evaluation type set to Equal.

In the end this is how the logic bricks should look.